446 
Marine Cercariae 
Cercaria pur pur ae sp. inq. 
(Lebour, 1907 B, p. 102.) 
(PI. XXIX, figs. 1-3.) 
This cercaria which I have before described from the dog-whelk 
Purpura kqnllvs undoubtedly belongs to the Echinostornum group 
although certainly to another genus. Having found it much more 
frequently than before I am able to describe it more in detail. It 
occurs in the digestive gland of Puipura lapillus from Budle Bay, 
Fenham Flats, Loch Ryan and Millport. It is the commonest at 
Millport in about 20 per cent, in spring, summer, and early autumn, 
but not in the winter. The infected gland is swollen and of a pale 
yellowish colour, packed full of rediae. The redia is pale yellow or 
colourless with a bright yellow intestine. The young forms are the 
most perfectly typical rediae that I know. Thejmneasure 0’32-0‘60 mm. 
in length and are narrow and quite colourless. The pharynx is 
conspicuous and leads to a large intestine occupying about half the 
length of the body. Anteriorly and to the side is a thick-lipped 
birth-pore which is capable of protrusion or withdrawal. A collar is 
not present. Two very mobile ambulatory processes occur posteriorly 
one on each side of the pointed hind end, and these are also 
capable of being retracted completely or exserted for quite a long 
distance. These young rediae are to be found in the mantle cavity 
and also in the digestive gland and most probably migrate from one 
organ to the other. The sporocyst in all probability occurs in the 
mantle cavity and gives rise to rediae which migrate to the digestive 
gland where they feed and multiply enormously, finally giving rise to 
cercariae. The older redia loses its hind appendages but its pharynx 
and intestine are still conspicuous when it is full of cei’cariae. The 
full-grown redia may be 3'6 mm. long. The cercaria is very solid and 
thick but extremely active. Length of body 0'64 mm. Avithout tail 
which is 0'42 mm. long. The tail is very thin with a sucker-like end. 
There is no indication of a continuation into it of the excretory system. 
Body entirely covered with broad scale-like spines except in the head 
region and the region of the ventral sucker. The inside of the body 
is occupied by large gland cells arranged in four groups, the ducts from 
which run together on each side expanding into six smaller ducts on 
each side which open by the oral sucker. The head is extremely 
complicated and beset with numerous spines and papillae. There is 
a considerable space, the neck, between the flat body spines and the 
